Nitride based semiconductor laser diode device with a bar mask

ABSTRACT

A nitride based semiconductor laser diode device comprising a selective growth mask with a grating structure is proposed. The island-like stacked epitaxial layers including the P-type cladding layer is formed from the selective growth mask upon the active layer of the semiconductor laser structure. This proposed structure can reduce the strain by the deformation due to the isolate structure. Thus, increase of thickness of the cladding layer and/or increase of composition difference can be achieved without crack existing in the island-like stacked epitaxial layers. The optical confinement can be effectively improved.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of a prior application Ser. No.10/250,200, filed Jun. 12, 2003, now U.S. Pat. No. 7,126,971 whichclaims the priority benefit of Taiwan application serial no. 91121531,filed on Sep. 20, 2002. All disclosure of the Taiwan application isincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of Invention

The present invention relates to a semiconductor device by galliumnitride compound. More particularly, the present invention relates to astructure capable of applying on a gallium nitride laser diode devicefor increasing a thickness of a surface epitaxial layer. In thespecification, the surface epitaxial layer is referring to a patternedepitaxial layer in the gallium nitride laser diode device.

2. Description of Related Art

The typical optoelectronics device is a hetero structure. Since thecrystal structure between the hetero epitaxial layers is not matched andthe thermal expansion coefficients are also different, a strain energyexists at the interface. During the fabrication process or in operationof the device, this strain energy will be partially released by a formof dislocation or other defect forms. When the thickness of theepitaxial layer is greater than a certain critical value, the materialthen release the energy by a crack form, resulting in a crack of theepitaxial layer. For this consideration, the hetero epitaxial crystallayer cannot be grown in overlarge thickness. If the thickness of theepitaxial layer is too large, a poor function of the device or even asevere damage would occur due to the crack. However, if the thickness ofthe epitaxial layer is not sufficient large, a poor performance may alsooccur.

In the conventional technology, it has several methods can reduce theproblems caused by the strain energy in the hetero structure. Forexample, (1) a method of epitaxial lateral overgrowth (ELOG) isproposed. In addition, (2) a buffer layer is also introduced, wherein analuminum nitride layer or a gallium nitride layer in a rather smallthickness is formed on a substrate at low temperature, to serve as abuffer layer, so as to reduce the problem of not being match for thecrystal lattice between the epitaxial layer and the substrate. As aresult, the condition for subsequently growing the gallium nitride athigh temperature is improved and the quality of epitaxial crystalstructure is also improved. In addition, (3) a strain layer superlatticeis also proposed.

The conventional gallium nitride device in the current status is mostlyusing the C-plane of the sapphire (Al₂O₃) as the substrate. It has aboutan amount of 16% in lattice mismatch existing between the substrate andthe gallium nitride epitaxial crystal, resulting in a rather largestrain energy inside the gallium nitride thin film, which is grown onthe sapphire, in which the density of the dislocation is high up to10⁹–10¹¹/cm². The foregoing technology can only be used for solving thestrain energy or effect, which are caused by epitaxial layer betweensubstrate and the gallium nitride, or inside the device.

Taking the gallium nitride laser diode as an example, the applicationfor the foregoing conventional technologies is basically limited to theepitaxial layer under the active layer. The epitaxial layer above theactive layer still has the problem of hetero material structure. Inother words, the conventional method still cannot effectively solve theproblem of crack in the epitaxial layer above the active layer.

FIG. 1 is a cross-sectional view, schematically illustrating thestructure of a conventional gallium nitride laser diode, which issequentially formed with a substrate 101, a buffer layer 102 withgallium nitride compound semiconductor formed at relative lowtemperature, an N-type gallium nitride compound semiconductor layer 103,a set of bar mask 104, an N-type gallium nitride compound semiconductorlayer 105, a heavily doped layer 106, an N-type gallium nitride compoundsemiconductor superlattice cladding layer 107, an N-type gallium nitridecompound semiconductor light guiding layer 108, a gallium nitridecompound semiconductor active layer 109, a P-type gallium nitridecompound semiconductor cap layer 110, a P-type gallium nitride compoundsemiconductor light guiding layer 111, a P-type gallium nitride compoundsemiconductor superlattice cladding layer 112, and a P-type metalelectrode contact layer 113. In order to improve the quality of theepitaxial crystal and prevent the crack from occurring, the conventionalstructure for the gallium nitride laser diode structure has included thelow-temperature buffer layer 102, the method of ELOG (104), the strainlayer super lattice structure (107, 112). However, these kinds oftechnologies for reducing the strain and the defects is not effectivewith respect to strain energy existing in the surface hetero epitaxiallayer. FIG. 2, is a picture, schematically illustrating the crackoccurring on the surface of the epitaxial layer for the conventionalgallium nitride laser diode structure.

On the gallium nitride laser diode structure, the cladding layer isusually formed from Al_(x)Ga_(1−x)N, where if the quantity of X ishigher, then the refraction index is smaller, the energy gap is larger,and the light confinement is better. As a result, the lattice mismatchis larger, thus, the thickness can not be overlarge. If theAl_(x)Ga_(1−x)N layer is too thick, the crack is then easily occurring,and causes a failure of device. However, if the cladding layer is notsufficiently thick, the effect of light confinement then is gettingworse, and the performance of device is then poor. Thus, it is adifficult issue for fabrication that how to control the thickness ofepitaxial layer and the composition, so as to reduce the cracking of theepitaxial layer and improve the performance of the device.

SUMMARY OF THE INVENTION

The invention provides an epitaxial growing structure, which uses aspecific mechanism for the surface, so as to increase the thickness ofthe epitaxial layer in the gallium nitride laser diode structure butreduce the occurrence of crack.

As embodied and broadly described herein, the invention provides a laserdiode structure with an epitaxial crystal growing structure, whichcomprises a P-type gallium nitride compound semiconductor light guidinglayer, formed on a surface of the active layer. A set of bar masksubstance is formed on the P-type gallium nitride compound semiconductorlight guiding layer. An island-like stacked structure is formed on theP-type gallium nitride compound semiconductor light guiding layer andthe set of bar mask substance. The island-like stacked structure isformed including a P-type gallium nitride compound semiconductorcladding layer and a P-type metal electrode contact layer.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary, and are intended toprovide further explanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide a furtherunderstanding of the invention, and are incorporated in and constitute apart of this specification. The drawings illustrate embodiments of theinvention and, together with the description, serve to explain theprinciples of the invention. In the drawings,

FIG. 1 is a cross-sectional view, schematically illustrating thestructure of a conventional gallium nitride laser diode;

FIG. 2 is a picture, schematically illustrating the crack occurring onthe surface of the epitaxial layer for the conventional gallium nitridelaser diode structure;

FIG. 3 is a cross-sectional view, schematically illustrating thestructure of a gallium nitride laser diode, according to the firstpreferred embodiment of this invention;

FIG. 4 is a top view, schematically illustrating the structure of thebar mask substance 313 over the epitaxial layer, according to the firstpreferred embodiment of this invention;

FIG. 5 is a cross-sectional view, schematically illustrating thestructure of a gallium nitride laser diode, according to the secondpreferred embodiment of this invention;

FIG. 6 is a cross-sectional view, schematically illustrating thestructure of a gallium nitride laser diode, according to the thirdpreferred embodiment of this invention;

FIG. 7 is a cross-sectional view, schematically illustrating thestructure of a gallium nitride laser diode, according to the fourthpreferred embodiment of this invention; and

FIG. 8 is a cross-sectional view, schematically illustrating therelative position for the lower bar mask substance, the upper bar masksubstance, the active layer, and the island-like stacked structure,according to the preferred embodiment of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Several terms used in the specification of the invention are generallydescribed as follows. The term of gallium nitride compound semiconductor(GNCS) is referring to a form of Al_(x)In_(y)Ga_((1−x−y))N (0≦x≦1,0≦y≦1, 0≦x+y≦1), which can be formed by, for example, metal organicchemical vapor deposition (MOCVD) or molecular beam epitaxial (MBE)crystal growth, or other similar epitaxial crystal growth. The dopantsdoped in the N-type gallium nitride compound semiconductor of theinvention include Si, Ge, or other like elements with the same function.The dopants doped in the P-type gallium nitride compound semiconductorof the invention include Mg, Zn, Be, or other like elements with thesame function.

Embodiment 1

FIG. 3 is a cross-sectional view, schematically illustrating thestructure of a gallium nitride laser diode, according to the firstpreferred embodiment of this invention. A substrate 301 is provided. Thesubstrate 301 includes, for example, sapphire (Al₂O₃), silicon carbide(SiC), gallium nitride (GaN), Spinel (MgAl₂O₄), gallium arsenide (GaAs),zinc oxide (ZnO), silicon (Si), and so on. A gallium nitride compoundsemiconductor buffer layer 302 is formed on substrate 301 with anamorphous and/or polycrystallized structure and a thickness of 50–500Angstrom. An N-type GNCS 303 is formed on the buffer layer 302 with athickness of about 3–7 microns.

After the crystal growth, the whole structure is shifted out from thecrystal growing machine, and the coating, photolithography and etchingprocesses are performed to form a lower bar mask substance 304 on theN-type GNCS layer 303. The direction of the bar can be any direction.The thickness can be, for example, 100–2000 Angstroms and the width canbe, for example, 2–20 microns. The distance between the parallel bars inthe bar mask substance 304 is about 2–500 microns. The bar masksubstance 304 is used to cause multiple local crystal growing regionsduring the regrowth on the epitaxial layer, so as to change thedistribution of strain force in the epitaxial layer and the growingdirection of dislocation. As a result, the epitaxial layer formed overthe N-type GNCS layer 303 has less dislocation density and strainenergy. The bar mask substance 304 by itself has to have sufficientstability without producing the chemical reaction with the peripheralepitaxial materials, or contaminating the peripheral epitaxialmaterials. In addition, during the process of epitaxial growing, theepitaxy of GNCS does not directly grow on the mask substance. Each barof the lower bar mask substance 304 has a cross-sectional shape inrectangular or any shape with similar functions. The mask substance 304preferably includes silicon oxide, silicon nitride, or other materialwith similar function.

Then, the substrate 301 is transferred back to the epitaxial growingmachine. An N-type GNCS layer 305 is formed on the surface of the N-typeGNCS layer 303 and the lower bar mask substance 304. The N-type GNCSlayer 305 is grown to have a planar surface and a thickness of 3–20microns. A heavily doped N-type GNCS layer 306 is formed on the N-typeGNCS layer 305 with a thickness of, for example, between 500 Angstromsand 2 microns. An N-type GNCS cladding layer 307 is formed on theheavily doped N-type GNCS layer 306. The N-type GNCS cladding layer 307is a multi-layer structure, including a super lattice structure composedof N-type gallium nitride semiconductor layer/N-type aluminum galliumnitride semiconductor layer with a thickness of 35–200/35–200 Angstroms.Moreover, the pair number is about 3 to 100 pairs. As a result, thetotal thickness is about 210–40000 Angstroms. An N-type GNCS lightguiding layer 308 is formed on the N-type GNCS cladding layer 307 with athickness of about 100–2000 Angstroms. Even though this layer can bedoped with N-type dopants, it can also be non-doped. A GNCS active layer309 is formed on the GNCS light guiding layer 308 with a single-layerstructure or a multi-layer structure by a thickness of about 30 –1000Angstroms. A P-type GNCS light guiding layer 310 is formed on the GNCSactive layer 309 by a thickness of about 100–2000 Angstroms. Even thoughthis layer can be doped with P-type dopants, it can also be non-doped. AP-type GNCS cap layer 311 is formed on the GNCS light guiding layer 310by a thickness of about 300–2000 Angstroms. A P-type GNCS light guidinglayer 312 is formed on the GNCS cap layer 311 by a thickness of about200–3000 Angstroms.

Further still, the whole structure is transferred out from the epitaxialgrowing machine. The coating, photolithography, and etching processesare performed to form an upper bar mask substance 313 on the P-type GNCSlight guiding layer 312. The direction of the bars should be the same asthe direction of the lower bar mask substance 304. The thickness of thebars is about 100–2000 Angstroms and the width is about 50–500 microns.The distance between the parallel bars is about 1–10 microns. Similar tothe lower bar mask substance 304, the upper bar mask substance 313 byitself should have sufficient stability without producing the chemicalreaction with the peripheral epitaxial materials, or contaminating theperipheral epitaxial materials. In addition, during the process ofepitaxial growing, the epitaxy of GNCS does not directly grow on themask substance. Each bar of the upper bar mask substance 313 has across-sectional shape in rectangular or any shape with similarfunctions. The mask substance 313 preferably includes silicon oxide,silicon nitride, or other material with similar function.

FIG. 4 is a top view, schematically illustrating the structure of thebar mask substance 313 over the epitaxial layer. The upper bar masksubstance 313 is used to form the island-like stacked structure on thesurface of the gallium nitride laser diode. Thereby, the absorption ofthe elastic strain from the material can release the strain energyinside the epitaxial crystal. As a result, the epitaxial layer over thebar mask substance 313 can be grown by a larger thickness without crack.This is one of the advantages of the invention. Further descriptions areas follows.

The whole structure is then transferred back to the epitaxial growingmachine. A P-type GNCS cladding layer 314 is formed on the P-type GNCSlight guiding layer 312 and the upper bar mask substance 313. Since theupper bar mask substance 313 produces a crystal growing selectiveeffect, the P-type GNCS cladding layer 314 is grown as island-likestacked structure. In order to allow the elastic strain to be effective,the islands to each other should be separated without connection. TheP-type GNCS cladding layer 314 can be a single layer of aluminum galliumnitride semiconductor or a composed layer of P-type gallium nitridesemiconductor/P-type aluminum gallium nitride semiconductor with thesuper lattice structure. The total thickness is about 0.5–10 microns. AP-type metal electrode contact layer 315 is formed on P-type GNCScladding layer 314 by a thickness of about 150–2000 Angstroms.

Embodiment 2

FIG. 5 is a cross-sectional view, schematically illustrating thestructure of a gallium nitride laser diode, which includes a substrate501 having the materials including aluminum oxide, silicon carbide,gallium nitride, Spinel, gallium arsenide, zinc oxide, silicon, and soon. A GNCS buffer layer 502 is formed on the substrate 501 as anamorphous and/or polycrystallized structure by a thickness of 50–500Angstroms. An N-type GNCS layer 503 is formed on the GNCS buffer layer502 by a thickness of about 3–7 microns.

After the crystal growth, the whole structure is shifted out from thecrystal growing machine, and the coating, photolithography and etchingprocesses are performed to form a lower bar mask substance 504 on theN-type GNCS layer 503. The features of the lower bar mask substance 504is similar to the lower bar mask substance 304 in Embodiment 1. Each barof the lower bar mask substance 504 has a cross-sectional shape inrectangular or any shape with similar functions. The direction of thebar can be any direction. The thickness can be, for example, 100–2000Angstroms and the width can be, for example, 2–20 microns. The distancebetween the parallel bars in the bar mask substance 504 is about 2–500microns. The material includes, for example, silicon oxide, siliconnitride, or other material with similar function.

Then, whole structure is transferred back to the epitaxial growingmachine. An N-type GNCS layer 505 is formed on the surface of the N-typeGNCS layer 503 and the lower bar mask substance 504. The N-type GNCSlayer 505 is grown to have a planar surface and a thickness of 3–20microns. A heavily doped N-type GNCS layer 506 is formed on the N-typeGNCS layer 505 with a thickness of, for example, between 500 Angstromsand 2 microns. An N-type GNCS cladding layer 507 is formed on theheavily doped N-type GNCS layer 506. The N-type GNCS cladding layer 507is a multi-layer structure, including a super lattice structure composedof N-type gallium nitride semiconductor layer/N-type aluminum galliumnitride semiconductor layer with a thickness of 35–200/35–200 Angstroms.Moreover, the pair number is about 3 to 100 pairs. As a result, thetotal thickness is about 210–40000 Angstroms. An N-type GNCS lightguiding layer 508 is formed on the N-type GNCS cladding layer 507 with athickness of about 100–2000 Angstroms. Even though this layer can bedoped with N-type dopants, it can also be non-doped. A GNCS active layer509 is formed on the GNCS light guiding layer 508 with a single-layerstructure or a multi-layer structure by a thickness of about 30–1000Angstroms. A P-type GNCS light guiding layer 510 is formed on the GNCSactive layer 509 by a thickness of about 100–2000 Angstroms. Even thoughthis layer can be doped with P-type dopants, it can also be non-doped. AP-type GNCS cap layer 511 is formed on the GNCS light guiding layer 510by a thickness of about 300–2000 Angstroms. A P-type GNCS light guidinglayer 512 is formed on the GNCS cap layer 511 by a thickness of about200–3000 Angstroms. A P-type GNCS cladding layer 513 is formed on theP-type GNCS light guiding layer 512. The P-type GNCS cladding layer 513is a multi-layer structure composed of P-type gallium nitridesemiconductor layer/P-type aluminum gallium nitride semiconductor layerby thickness of 35–200/35–200 Angstroms, and the pair number is about 3to 10, resulting in a total thickness of about 210–4000 Angstroms.

Then, the whole structure is transferred out from the epitaxial growingmachine. The coating, photolithography, and etching processes areperformed to form an upper bar mask substance 514 on the P-type GNCScladding layer 513. The features of the bars in the upper bar masksubstance 514 are similar to those in the bar mask substance 313 inEmbodiment 1 with the cross-sectional shape in rectangular or any shapewith similar functions. The direction of the bars should be the same asthe direction of the lower bar mask substance 504. The thickness of thebars is about 100–2000 Angstroms and the width is about 50–500 microns.The distance between the parallel bars is about 1–10 microns.

The whole structure is transferred back to the epitaxial growingmachine. A P-type GNCS cladding layer 515 is formed on the P-type GNCScladding layer 513 and the upper bar mask substance 514. Since the upperbar mask substance 514 produces a crystal growing selective effect, theP-type GNCS cladding layer 515 is grown as island-like stackedstructure. The islands to each other should be separated independentlywithout connection, so as to have the elastic strain effect. The P-typeGNCS cladding layer 515 can be a single layer of aluminum galliumnitride semiconductor or a composed layer of P-type gallium nitridesemiconductor/P-type aluminum gallium nitride semiconductor with thesuper lattice structure. The total thickness is about 0.5–10 microns. AP-type metal electrode contact layer 516 is formed on P-type GNCScladding layer 515 by a thickness of about 150–2000 Angstroms.

Embodiment 3

FIG. 6 is a cross-sectional view, schematically illustrating thestructure of a gallium nitride laser diode, according to the thirdpreferred embodiment of this invention. The structure is formed byphotolithography, etching, and coating processes on the structure ofgallium nitride laser diode (see FIG. 3) in Embodiment 1. In addition tothe gallium nitride laser diode in epitaxial structure, the structurefurther includes an insulating layer 601 from, for example, siliconoxide. A P-type metal electrode 602 is formed on the P-type metalelectrode contact layer 315. An N-type electrode 603 is formed on theheavily doped N-type GNCS layer 306.

Embodiment 4

FIG. 7 is a cross-sectional view, schematically illustrating thestructure of a gallium nitride laser diode, according to the fourthpreferred embodiment of this invention. The structure is formed byphotolithography, etching, and coating processes on the structure ofgallium nitride laser diode (see FIG. 5) in Embodiment 2. In addition tothe gallium nitride laser diode in epitaxial structure, the structurefurther includes an insulating layer 701 from, for example, siliconoxide. A P-type metal electrode 702 is formed on the P-type metalelectrode contact layer 516. An N-type electrode 703 is formed on theheavily doped N-type GNCS layer 506.

If the lower bar mask substance 304, 504 are properly associating withthe upper bar mask substance 313, 514, then the gallium nitride laserdiode can have better efficiency. FIG. 8 is a cross-sectional view,schematically illustrating the relative position for the lower bar masksubstance, the upper bar mask substance, the active layer, and theisland-like stacked structure, according to the preferred embodiment ofthis invention. In FIG. 8, “A” represents the island-like stackedstructure. B1 represents the upper bar mask substance at a left side ofthe island-like stacked structure A. B2 represents the upper bar masksubstance at a right side of the island-like stacked structure A. Crepresents the active layer. D represents the lower bar mask substance.E represents the substrate. The dashed line 1 represents the center lineof the short side of the lower bar mask substance. The dashed line 2represents the center line of the window region between the bars of thelower bar mask substance. “b10” represents a side of the upper bar masksubstance B1 close to the island-like stacked structure A. “b20”represents a side of the upper bar mask substance B1 close to theisland-like stacked structure A. The relative locations for the galliumnitride laser diode structure are described as follows.

1. The upper bar mask substance B1, B2 are located between theisland-like stacked structure A and the active layer C.

2. The lower bar mask substance D is located between the substrate E andthe active layer C.

3. The side b10 of the upper bar mask substance B1 has to cross over thecenter line 1 (dashed line 1) of the short side of the lower bar masksubstance D.

4. The side b20 of the upper bar mask substance B2 has to cross over thecenter line 2 (dashed line 2) of the window region between the bars ofthe lower bar mask substance.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the structure of the presentinvention without departing from the scope or spirit of the invention.In view of the foregoing, it is intended that the present inventioncovers modifications and variations of this invention provided they fallwithin the scope of the following claims and their equivalents.

1. A gallium nitride laser diode, comprising: a substrate; and amulti-layer structure for the gallium nitride laser diode, furtherincluding a lower bar mask substance, an N-type gallium nitride compoundsemiconductor (GNCS) cladding layer, an active layer, an upper bar masksubstance, and a P-type gallium nitride compound semiconductor claddinglayer, sequentially formed over the substrate.
 2. The gallium nitridelaser diode of claim 1, wherein the upper bar mask substance is locatedover the active layer.
 3. The gallium nitride laser diode of claim 1,wherein bars of the upper bar mask substance are parallel and have adesired direction, wherein the bars have a thickness of about 100–2000Angstroms and a width of 50–500 microns, and a distance between theparallel bars is about 1–10 microns.
 4. The gallium nitride laser diodeof claim 1, wherein directions the upper bar mask substance and thelower mask substance are parallel to each other.